Tyrannosaurid dinosaurs were dominant predators reaching extraordinary body sizes, but they definitively only occupied this niche for the last two geological stages (83-65 MYA) of the Mesozoic. It has recently been hypothesized that another theropod clade, the allosauroids, competitively excluded tyrannosauroids from the top predator niches for most of the Jurassic and Cretaceous. The theropod fossil record is relatively poor, however, and support for this hypothesis relies on the temporal and geographic distribution of a handful of body fossils. Theropod teeth are relatively abundant, however, as they were replaced throughout life, and therefore offer a richer source of data with which to test this hypothesis. The museum has collections of theropod teeth from three American Cretaceous formations ranging from >112 to 98 million years ago.
Research methods and techniques: Using the museum’s SEM, an REU intern will analyze microstructural characters of the enamel of select teeth from each formation, along with gross morphological characters for a wider sample, and use these traits to assign the teeth to different theropod clades based on synapomorphies. The size distributions of teeth referable to allosauroids and tyrannosauroids will be compared to determine whether they support the competitive exclusion hypothesis. Statistical approaches to determining minimum numbers of individuals and to account for sampling of juveniles will be applied to determine the robustness of the comparisons between clades.
Curator/Advisor: Dr. Peter Makovicky (Geology)
REU Intern: FRANCO GALLASTEGUI
Biology and Geophysical Sciences major
University of Chicago
Symposium Presentation Title: Using Systematic Traits of Teeth to Test Trophic Niche Evolution in Theropod Dinosaurs
Symposium Presentation Abstract: Tyrannosaurids are the subclade of coelurosaurian dinosaurs that include T. rex and its closest relatives. While notorious for achieving colossal body sizes, tyrannosaurids evolved from small- bodied Asian precursors and did not become a dominant predator until the latest stages of the Mesozoic (Campanian and Maastrichtian). Based on observations from the Asian record, it has been suggested that allosauroids competitively excluded tyrannosauroids from the top-predator niches in North America during most of Cretaceous. Support for this hypothesis, however, consists of a small number of temporally and geographically disparate body fossils. Theropod teeth, because they are abundant across several well-correlated sections in NA, provide a richer source of data for which to test this hypothesis. In addition, studies have established that the 3D arrangement of enamel crystallites (the schmelzmuster) is a character that exhibits a strong phylogenetic signal in theropods, which allows for isolated teeth to be taxonomically diagnosed with family and even genus-level precision. In this study, I measured variables pertaining to size, shape, and serration density for a number of tooth samples from three Lower Cretaceous formations in western NA. In addition, I coded the teeth for several morphological characters used in a previous analysis of theropod phylogeny. Since enamel visualization requires destructive sampling, representatives were chosen from clusters based on morphological similarity as well as on rough taxonomic affinity. Teeth were sectioned in longitudinal and transverse planes and their enamel examined using scanning electron microscopy. Together, the microstructural and gross morphological characters will be used to assign teeth to different theropod groups. The size distribution of teeth referable to allosauroids and tyrannosauroids will be compared to determine whether they support the competitive exclusion hypothesis.